The present invention relates to a process for the production of powders of metal oxides or metal mixed oxides with excellent fluidity and high bulk density. In this process, metal salt solutions are broken down, with the application of ultrasonic waves, into fine droplets, which are then converted into stable metal hydroxide gel particles in an atmosphere of ammonia gas. These solidified gel particles are collected and, after drying, are converted into the corresponding metal oxides or metal mixed oxides.
It is already known (German Auslegeschrift No. 2,459,445) to break down thorium or uranium salt solutions, by means of a liquid jet displaced into oscillation, into fine drops, and then to partially solidify these drops by precipitation of the hydroxides in an atmosphere of ammonia gas, before they penetrate into an ammonia solution. The spherules converted into the oxides have diameters of 200-500 .mu.m.
According to another process (German Offenlegungsschrift No. 3,035,331), spherical particles which are especially suitable for use in nuclear technology may be produced with diameters between 50 and 2500 .mu.m and a narrow grain spectrum. In this process, solutions or melts are converted, by means of a vibrator system at frequencies of 50-2500 Hz, into drops which are subsequently solidified chemically or physically to form spherical particles. The drop or particle size is dependent upon the frequency of the vibrator and the throughput rate of the liquid.
Other processes are also known for the solidification of the droplets, in which processes the drops are solidified either by dehydration (sol-gel process according to U.S. patent specification No. 3,290,122) or by reaction of polymeric additives with an alkaline medium (German Patent Specification No. 1,212,841). A feature common to all these processes is the introduction of the - in part already "presolidified" - drops into a fluid phase, in which the solidification is then completed.
All these processes exhibit disadvantages which reside in the attainable minimum grain size (grain sizes representing diameters of only a few .mu.m cannot be produced), the relatively broad grain spectrum or in the interposition of, in most cases, organic barrier liquids, on passing through which the drops can absorb extraneous substances, which thus pass into the products. Moreover, these--in most cases organic--phases are consumed by processes of adsorption at the drops, so that they must constantly be renewed. A further disadvantage in the application of fluid barrier phases consists in the water enrichment, proceeding together with the dehydration in the drop, in the alkaline phase. In order to be able to operate the process over a longer period of time under constant conditions, it is necessary to provide regeneration circuits, in which the proportion of water is again reduced.